A piezoelectric speaker converts an electric signal into sound using as a vibrator a piezoelectric element that deforms when a voltage is applied thereto. The sound includes an alarm, a melody, and the like, as well as voice. Hereinafter, description is made exemplifying sound as voice.
In a piezoelectric speaker, a circular piezoelectric element is attached to a diaphragm held in the piezoelectric speaker. When an electric signal is applied to the piezoelectric element, the piezoelectric element deforms in response to the applied electric signal. The electric signal to be applied has a voltage and a current changed variously depending upon an input voice signal, and thus the deformation of the piezoelectric element results in deformation motion in accordance with a change in input voice signal. The deformation motion of the piezoelectric element is transmitted to the diaphragm with the piezoelectric element attached thereto, and the vibration of the diaphragm vibrates surrounding air, whereby voice is generated.
It is desired that the diaphragm of the piezoelectric speaker have a largest possible vibration plane and be capable of vibrating freely with a largest possible amplitude. This is because, if the diaphragm is capable of vibrating largely due to a large vibration plane, voice with a large volume under a large sound pressure can be generated. Further, if the diaphragm is capable of vibrating freely, the deformation motion of the piezoelectric element is converted into vibration motion efficiently, which enables a signal input to the piezoelectric element to be reproduced more accurately.
Regarding a method of supporting a diaphragm, various methods are proposed. More specifically, a support member, a support structure, and the like of the diaphragm are proposed (see, for example, Patent Document 1: JP 2005-130156 A, Patent Document 2: JP 2001-119795 A, Patent Document 3: JP 10-164694 A).
Referring to FIG. 1, an example of a support structure of the diaphragm will be described.
In the support structure of the diaphragm shown in FIG. 1, a diaphragm 11 is supported between a housing 12 and a structural part (a cover 13) inside the piezoelectric speaker via spacers 14 and 15. By using members having appropriate elasticity such as silicone rubber, the diaphragm 11 is held at the structural part while keeping an appropriate degree of freedom.
However, when only a degree of freedom of vibration motion is pursued in the support structure of the diaphragm of the piezoelectric speaker, there arises a problem in that the sound characteristics of the piezoelectric speaker may be degraded partially. Specifically, problems such as the degradation in reproduction characteristics at a time of rising during the start of a signal input and the degradation in reproduction characteristics of a signal with a small sound pressure are caused.
Those problems are caused by the fact that the motion of the diaphragm for generating voice is reciprocating motion in a direction perpendicular to the vibration plane, whereas the deformation of the piezoelectric element is extension and contraction motion parallel to the plane of the diaphragm, and thus, those motions are in completely different directions.
Hereinafter, the mechanism of converting the extension and contraction motion of the piezoelectric element into the reciprocating motion in the direction perpendicular to the diaphragm in the piezoelectric speaker will be described.
When a voice signal is input to the piezoelectric speaker, an electric signal is applied to the piezoelectric element, and the piezoelectric element extends. When the piezoelectric element extends, the diaphragm is also extended along therewith.
Here, if the diaphragm is capable of extending by an extension amount or more of the piezoelectric element and is supported in an ideal state, and there is no factor for blocking the extension of the diaphragm, the diaphragm is extended freely in parallel with the vibration plane by the extension amount of the piezoelectric element. However, because the diaphragm is supported by the support structure, a reaction force is generated from the support structure when the diaphragm extends to some degree, whereby the extension is blocked. When the extension of the diaphragm is blocked, the motion of the diaphragm of spreading in a direction parallel to the vibration plane loses a place to go, with the result that the diaphragm starts bending. Due to the bending of the diaphragm, the extension of the piezoelectric element is converted into the motion in a direction perpendicular to the diaphragm. Once the diaphragm starts bending, the motion of the diaphragm gains momentum as the bending motion, and the extension and contraction motion of the piezoelectric element thereafter is converted into the bending motion of the diaphragm to become vibration motion.
In the piezoelectric speaker, the extension and contraction motion of the piezoelectric element is converted into the bending motion of the diaphragm through the above-mentioned process, whereby the diaphragm is vibrated finally. Further, in order for the diaphragm to start the vibration motion, a “trigger” for the diaphragm to initially start the bending motion is required.
The above-mentioned point will be described by way of a specific example shown in FIGS. 2A to 2D. FIGS. 2A to 2D are views showing that the piezoelectric speaker in a silent state (no input state) vibrates the diaphragm to start generating voice in four stages in accordance with a time passage.
The piezoelectric speaker includes a piezoelectric element 21, a diaphragm 22, and holding members 23, 24, 25, and 26. The piezoelectric element 21 is attached to the diaphragm 22 in intimate contact therewith so that the extension and contraction motion thereof is transmitted to the diaphragm 22. The holding members 23 to 26 are members having appropriate elasticity, such as silicone rubber, and each one end thereof is attached and fixed to a housing or a cover portion (hereinafter, described as a housing) (not shown). Further, each of the holding members 23 to 26 is bonded to the diaphragm 22 at the other end thereof. That is, the holding members 23 to 26 are positioned between the diaphragm 22 and the housing, thereby holding and fixing the diaphragm 22 to the housing. Simultaneously, due to the elasticity of the holding members 23 to 26, the holding and fixing of the diaphragm 22 is rendered flexible connection, thereby ensuring the degree of freedom at which the diaphragm 22 performs vibration motion to such a degree as to generate voice.
FIG. 2A shows a piezoelectric speaker in an initial state, which is stopped in a silent state, with no voice signal input thereto. Next, FIG. 2B shows a state in which a voice signal starts being input to the piezoelectric speaker and the piezoelectric element 21 starts extending. FIGS. 2B and 2C show that the piezoelectric element 21 extends and the diaphragm 22 also extends along therewith. In the stages shown in FIGS. 2B and 2C, the extension of the diaphragm 22 is absorbed by the deformation of the holding members 23 to 26. However, as the extension of the diaphragm 22 proceeds from FIG. 2B to FIG. 2C, reaction forces RF1 and RF2 from the holding members 23 to 26 also increase. In the stage shown in FIG. 2D, the reaction forces RF1 and RF2 from the holding members 23 to 26 become larger than the force by which the diaphragm 22 tries to extend, and the force of the diaphragm 22 of trying to extend, which is transmitted from the piezoelectric element 21, loses a place to go. The force in the extension direction, which has lost a place to go, causes the diaphragm 22 to bend so as to swell a center portion thereof, and escapes in a perpendicular direction. Thus, the diaphragm 22 starts bending. Thereafter, the extension and contraction motion of the piezoelectric element 21 is continuously converted into the bending motion of the diaphragm 22 to become the vibration motion of the diaphragm 22, whereby voice starts being generated.
As described above, in order for the extension and contraction motion of the piezoelectric element 21 to be converted into the bending and vibration motions of the diaphragm 22, the above-mentioned “trigger” for starting the conversion of the extension motion into the bending motion of the diaphragm 22 is required. Then, in order to allow the bending motion to start from the early stage in which the diaphragm 22 starts the extension operation, a structure of holding the diaphragm 22 with a strong binding force is desired. As the structure of holding the diaphragm 22 with a strong binding force, for example, the elasticity of the holding members 23 to 26 is reduced to be hard holding members in the example shown in FIGS. 2A to 2D. This is because the hard holding member with a small elasticity generates a large reaction force in response to even small deformation, and starts the bending of the diaphragm 22 in the stage in which the extension of the diaphragm 22 is small.
However, the support structure for holding the diaphragm 22 with a strong binding force, the “trigger” is obtained in the early stage, and the bending motion is started quickly. On the other hand, however, the motion of the diaphragm 22 after the start of the bending motion is also blocked, which impairs the sound characteristics of the piezoelectric speaker.
Conversely, when the degree of freedom of the vibration motion of the diaphragm 22 is enhanced considering the sound characteristics, the start of the bending motion in the diaphragm 22 is blocked. Specifically, the start of the vibration of the diaphragm 22 is delayed, whereby the start of the reproduction of voice at a time of the start of the input of a voice signal is delayed. That is, the rising of the voice reproduction becomes dull. Further, when the input of a voice signal is performed with a small volume and sound pressure (amplitude) from the start to the end, the deformation of the piezoelectric element 21 also becomes a small extension and contraction motion. Consequently, the bending motion of the diaphragm 22 does not start until the end, and voice may not be generated until the end.